1. Field of the Invention
This invention relates to a process for producing a copper powder that exhibits little agglomeration despite small particle diameter and to a copper powder enabling production of a pore-free sintered product at a low sintering temperature.
2. Background Art
Bonding or fixing conductive circuit members at desired locations on an insulating board is commonly done using a conductive paste. The conductive paste utilizes a powder of copper, nickel, silver or the like as its conductive material. Copper paste is widely used because copper powder is not only inexpensive but also low in electrical resistance and resistant to migration.
Recently it is proposed to form a terminal electrode of multi-layer ceramic capacitors by using an electrical conductive past having a metal powder therein as a filler. In this case, the conductive past is attached to the baked laminated ceramics of an dielectric substance, then heated as a whole at temperatures sufficient to occur the vaporization of vehicle component or the decomposition of the resins of the paste and to sinter the residual metal powder in the past thereby to form a terminal electrode. This conductive paste also generally uses copper powder as its powder component.
Known process of producing copper powder include the mechanical pulverization process, atomization process of spraying molten copper, electrolytic cathode deposition process, vapor deposition process and the wet reduction process. The wet reduction process is the main one used to produce copper powder for conductive paste because it is superior to the others in the point of enabling ready production of particles of uniformly small size. Copper powder production processes using the wet reduction method are taught by, for instance, Japanese Patent Publication JPA No. 4-116109 (1992), JPA No. 2-197012 (1990) and JPA No. 62-99406 (1987).
When the terminal electrodes of multi-layer ceramic capacitors are formed by a copper paste made from conventional copper powder, a sintering temperature higher than 800° C. is generally required to produce high-density conductors. This is because at a temperature of 800° C. or lower, the sintering that occurs, if any, does not integrally bind the particles to a degree sufficient to avoid a sintered body including many pores. It is therefore impossible to obtain good conductors. The need to employ a high sintering temperature exceeding 800° C. (under an inert atmosphere of 1 atmosphere) leads to the following problems.
When heated to a temperature higher than 800° C., multi-layered ceramics are, depending on their material, liable to deteriorate and decrease the capacity by incurring cracks between the ceramics and external electrode based on the shrinkage of the past as to cause mismatch with the ceramics. This restricts the selection of the ceramics material of the stacked boards.
Aside from these quality-related problems, high-temperature sintering is also costly in terms of energy and facility costs since it requires more heating energy, a longer heating period and more expensive heating equipment, thus increasing production cost. It also tends to lower yield.
The object of the present invention is therefore to provide a copper powder enabling production of a solid sintered product with few pores even at a low sintering temperature.